Type this:

CREATE NOCASE INDEX IF NOT EXISTS "ixFecha" ON "APACTU" 
("Codigo" ASC, "Activa" ASC ,"App" ASC ,"Fecha" DESC) COMPRESS FULL;

and DBISAM will create this: (reversed enginered using DBSYS)

CREATE NOCASE INDEX IF NOT EXISTS "ixFecha" ON "APACTU" ("Codigo"
DESC,"Activa","App","Fecha") COMPRESS FULL;